Many optical telecommunications devices require a low loss interface between a laser chip and a waveguide of a Photonic Integrated Circuit (PIC). In addition, it is desirable for this interface to be generally insensitive to alignment. A laser chip generally produces a small spot that is elliptical in shape, with an aspect ratio of between 2:1 and 3:1. Using one or more lenses, this divergent beam is collected and focused onto a PIC facet of the waveguide, the mode at the PIC facet having the same aspect ratio and size as the divergent beam focused by the one or more lenses; furthermore, the PIC facet of the waveguide is generally followed by a structure to taper the divergent beam received at the PIC facet down to a tightly confined waveguide mode used elsewhere on the PIC. The degree of spot size matching at the PIC facet and the loss induced by the mode conversion in the taper structure determine the overall coupler loss, which should be minimized.
Typical PIC edge couplers involve a simple waveguide taper, which does not provide independent control of spot size and mode asymmetry. For instance, the spot size of a confined mode in the waveguide is determined by the width of the waveguide at the end of the taper, with larger widths corresponding to smaller spot sizes. Alternatively, the ellipticity of the mode is determined by the ratio of the width of the waveguide to the thickness of the waveguide at the end of the taper, with more elliptical modes corresponding to waveguides with more rectangular cross-sections. Therefore, simple waveguide tapers are not appropriate to meet the demands of direct lens-coupling to a laser diode chip.
While sub-wavelength grating (SWG) waveguides can be used to reduce the waveguide index and produce larger spot size for a given waveguide width at a facet, they do not provide sufficient design flexibility.
Other technologies use local modifications of the refractive index and geometry of the cladding (the region surrounding the taper) to further control the spot size. However, this involves modifications to the standard foundry process and leads to compatibility issues with other structures.
Other solutions have used adiabatic mode spot-size converters, but these are intended for butt-joint (i.e. no lens) coupling, for example see N. Hatori, et al., “A novel spot size convertor for hybrid integrated light sources on photonics-electronics convergence system,” presented at the 8th IEEE Int. Conf. Group IV Photonics, San Diego, Calif., USA, Aug. 2012, Paper ThB2. For example, the device of Hatori produces a much smaller spot size and requires that the laser chip have close proximity to the PIC facet with extremely tight alignment tolerances, which generally requires flip-chip mounting, and leads to a coupling loss of 2.5 dB.